1. Field of the Invention
The present invention relates to digital images. More particularly, the present invention relates to processing digital images and denoising color digital images.
2. The Prior Art
The raw images created by digital cameras have to be transformed to standard color spaces such as sRGB (or Adobe RGB, or any other standard color space) by applying a color correction matrix. Typically, the color correction matrix increases saturation and consequently amplifies chroma noise. The trade-off between color saturation and noise amplification is well known. See Vora P. and Herley C, Trade-offs between color saturation and noise sensitivity in image sensors, Proceedings of the 1998 International Conference on Image Processing (1998). Therefore, to effectively reduce noise in a high ISO image, the noise amplification properties of the color correction matrix must be addressed.
If the color correction matrix is desaturated its condition number decreases and chroma noise amplification is reduced. The drawback of this simple approach is that in order to achieve reasonably low chroma noise in high ISO images, unacceptable levels of desaturation must be tolerated.
Multi-resolution approaches to denoising such as wavelet methods are commonly used in image processing. See, e.g., David Donoho, De-noising by soft thresholding, IEEE Trans, on Information Theory, 38(2), pp. 613-627, (1995), In these methods, typically, small coefficients in the high-frequency bands are set to zero.
In Fattal R., Agrawala M., and Rusinkiewicz S., Multiscale Shape and Detail Enhancement from Multi-light Image Collections, ACM SIGGRAPH 51 (2007), a multi-scale image decomposition is computed based on the bilateral filter. However, the purpose is not noise reduction.